1. Field of the Invention
The present invention relates to an electric steering system.
2. Description of the Related Art
Conventionally, for example, a method which applies a pulse voltage that does not cause a rotation of a rotor to a brushless motor is known when a magnetic-pole rotary angle of the rotor from a magnetic-pole position (i.e., a predetermined reference rotary position) in a stationary state of the brushless motor. In this method, a rotor angle (i.e., the magnetic-pole rotary angle of the rotor from the predetermined reference rotary position) indicating a rotor position is estimated on the basis of a variation in inductance in accordance with a salient-pole property of the rotor based on an inductance obtained by Fourier Transformation from a current and a voltage of the brushless motor (for example, see Non-Patent Document 1: YAMAMOTO et al., “Method of Estimating Initial Magnetic-Pole Position of Surface Magnet Synchronous Motor using Pulse Voltage”, IEEJ Trans. IA, Vol. 125, No. 3, 2005, p 253-258).
However, it is not possible to distinguish N and S poles by observing the inductance variation. For this reason, as shown in FIG. 4 of Non-Patent Document 1, the inductance varies by two amplitudes for each electric angle. This inductance variation inevitably occurs in a case where the rotor position is estimated on the basis of the inductance variation as well as a case of the method of Non-Patent Document 1.
When starting the motor in a state where the N and S poles are inverted by mistake, the brushless motor may rotate in a reverse direction. For this reason, for example, a method which supplies a current saturating a magnetic circuit so as to determine N and S poles of a rotor and a starting direction thereof based on an inductance being dissymmetric with respect to the N and S poles of the rotor in accordance with the magnetic saturation is known (for example, see Non-Patent Document 2: KANEKO et al., “Method of Estimating Initial Position of IPM Motor in Stationary State”, IEEJ Trans. IA, Vol. 132, No. 2, 2003, p 140-148).
However, in the above-described method according to an example of the related art (Non-Patent Document 2), since a large current saturating the magnetic circuit is excessively supplied in order to determine the magnetic pole of the rotor, an unnecessary torque is generated in the brushless motor. For this reason, in the brushless motor provided in the electric steering system, a problem arises in that a driver feels an discomfort due to unexpected steering torque, vibration, or noise.
Additionally, it is necessary to carry out a complex process such as a PLL process. From this point, a problem arises in that it is difficult to reduce a time necessary for determining polarity and it is not possible to improve responsiveness upon starting the brushless motor. In the brushless motor provided in the electric steering system, a steering assisting force generated from the brushless motor cannot appropriately assist a driver's steering input, thereby causing a problem in that a driver feels discomfort.
The present invention is contrived in consideration of the above-described circumstances, and an object of the invention is to provide an electric steering system capable of promptly starting a brushless motor being in a stationary state and of preventing a driver from feeling discomfort when steering while ensuring desired low noise.